1. Field of the Invention
The present invention relates to a novel polymerizable compound having a biphenyl skeleton, a polymerizable composition comprising the polymerizable compound, an optical material using the polymerizable composition, an optical element using the optical material, and an optical head device.
2. Discussion of Background
Resin materials having a high refractive index are widely used as optical materials, since optical elements may thereby be made to be small-sized or to have high efficiency. Specifically, they are used, for example, for display panels, eyeglass lenses, optical disks or optical filters.
In recent years, in such applications, it has been promoted to increase the light intensity or to shorten the wavelength to be used, and accordingly, it has been required to improve the light resistance of the high refractive index resin materials.
In a case where an optical material such as a high refractive index resin material is inferior in the light resistance, if such an optical material is used as a component or an element, a decrease in the transmittance or an increase in the optical strain is likely to occur as the time passes, and it becomes difficult to use it stably over a long period of time.
The above requirement is particularly distinct in the field of optical disks. For example, pits are formed by convexoconcaves provided on the surface of an optical disk such as CD (compact disk) or DVD (digital versatile disk). Information recorded in pits can be read by irradiating the optical disk surface with a laser light and detecting its reflected light by an optical head device. In recent years, in order to increase the capacity of an optical disk, it has been promoted to further shorten the wavelength of the laser light and to further reduce the pit size on the optical disk. For example, a system may be proposed wherein a laser light having a wavelength of from 300 to 450 nm (hereinafter referred to also as a blue laser light) is employed as a light source, and particularly BD (blu-ray disk) is remarkably growing in recent years.
An optical material to be used for such an optical head device is required to have not only high light resistance but also a high refractive index. For example, a polarization hologram element is used as a polarization separation element for an optical head device and has a structure wherein a birefringent resin and an isotropic resin are laminated. In the case of the polarization hologram element, the refractive index in ordinary ray direction or the refractive index in extraordinary ray direction of the birefringent resin is made to be substantially equal to the refractive index of the isotropic resin, in order to improve the polarization characteristics and the diffraction efficiency.
However, a high refractive index material usually has a large wavelength dispersion of the refractive index, and the absorption for short wavelength light tends to be large. Accordingly, in a case where a birefringent resin having a high refractive index of at least 1.55 is used for example, light resistance of the isotropic resin having a high refractive index matching therewith is not sufficient, and there has been a problem that deterioration in the transmittance is likely to occur.
Further, in the case of the after-mentioned wavelength-selective diffraction element, a material having a small wavelength dispersion of the refractive index (hereinafter referred to as a low wavelength dispersion property) and a material having a large wavelength dispersion of the refractive index (hereinafter referred to as large wavelength dispersion property) are laminated to form a diffraction grating.
In such a case, the larger the wavelength dispersion property of the large wavelength dispersion material, the more the wavelength-selective diffraction index difference with the low wavelength dispersion material can be made and the more the grating height can be reduced, whereby characteristics preferred in view of the production process and the diffraction efficiency will be obtained.
However, the large wavelength dispersion material is usually required to have an absorption band at the long wavelength side, and is likely to be inferior in the light resistance, like the above-described high refractive index material.
As the high refractive index resin material to be used as an optical material, heretofore, a compound having a skeleton of e.g. fluorene, tetraphenylmethane, 1,1,2,2-tetraphenylethane or biphenyl has been proposed (Patent Documents 1 and 2).
In the case of the compounds disclosed in Patent Documents 1 and 2, it is possible to improve the light resistance of the resin material to a certain extent by increasing the number of polymerizable groups in the molecule, by adding a light stabilizer, or by another means. However, even in such a case, sufficient light resistance is hardly obtainable, and further improvement in the light resistance is required.
On the other hand, Patent Document 3 discloses a silicon compound as a material which can satisfy both high refractive index and high light resistance. However, even the silicon compound disclosed in Patent Document 3 has insufficient light resistance and wavelength dispersion property, and it is required to improve the wavelength dispersion property while high light resistance is maintained.
However, a high refractive index and high light resistance, or large wavelength dispersion property and high light resistance, tend to conflict with each other, and it has been difficult to improve both of them. Further, it has been more difficult to improve all of the high refractive index, the large wavelength dispersion property and high light resistance to satisfactory levels.